Bees may come, and ants and other small creatures likely pollinate some too.

There's a blossom set spray, of doubtful efficacy, but you could give it a try if you're desperate.

I find the cantaloupes harder to hand pollinate than watermelons and I have a lot more experience on the later. What I've seen affect results with watermelon are time of day, temperature, sunshine, and humidity. The best time is early morning about an hour after sunrise as soon as the flowers open. If it was too cold at night the flowers may not open until later. Higher humidity and lack of wind keep the pollen damp enough to pick up on a small artist brush. If I can't see lots of pollen on the brush I've had no luck.

Even with the best of conditions I consider 10-20% a good rate of set fruit.

Not sure why the paint brush method isn't working. What kind of brush are you using? I use a very small soft brush, like the kind I used to paint plastic models as a kid. It works well. Melons are a bit fickle when it comes to pollinating but you should be able to get at least a few to set. Time of day is important. Afternoon is no good. I prefer morning after the sun has been up just long enough to dry things a little, maybe 9am or so when the pollen is fresh.